Semiconductor field effect transistors (FETs) continue to get smaller because of technological improvements in semiconductor fabrication processes. The technological improvements have enabled aggressive down-scaling of FETs, and the aggressive down-scaling has resulted in increased density of electrical components on integrated circuits. However, as FETs get smaller, challenges arise that can negatively impact their utility and performance.
One challenge often encountered in semiconductor fabrication, which arises due to down-scaling of FETs, is the ability to provide FETs with low contact resistance. A contact is an interface material between a FET substrate and interconnect wiring, wherein the interconnect wiring is routed to connect a FET to other integrated circuit components distributed on the surface of the substrate. Thus, contact resistance is the electrical resistance across the interface material, wherein the interface material typically comprises a silicide. A contact can enhance electrical current flow (i.e., reduce resistance) between substrate and interconnect wiring. However, as surface area of contacts decrease, due to the aggressive down-scaling, contact resistance can increase and cause a reduction of FET performance, such as a reduction in transistor switching speed.
A second challenge that arises in semiconductor fabrication, due to down-scaling of FETs, is leakage current. Field effect transistors are often utilized in low power applications, such as low leakage electronic devices that require minimal electrical power consumption. One important application for FETS is in battery operated low leakage electronic devices, wherein battery lifetime is essential.
Leakage power refers to the rate at which electrical energy is consumed by an electronic device that is on, but not performing useful work. Leakage power arises from leakage current that is inherent in FETs. Specifically, leakage current refers to current that flows through a FET when the FET is off but the electronic device utilizing the FET is on, which can result in electrical energy being dissipated even though the electronic device is not performing useful work. As FETs become smaller, the length of the transistor channel region (i.e., the region between the source and drain under the gates) becomes shorter, as well as distances from source and drain contacts to respective body region p-n junctions under the gate, which can result in increased leakage current and electrical energy dissipation. Generally, leakage current is undesirable because electrical energy (e.g., battery power) is dissipated by an electronic device without the device performing useful work. Providing low contact resistance and mitigating leakage current has become increasingly difficult to accomplish as the size of FETs become smaller.